The invention relates to the field of analog circuits, and in particular to the field of analog integrator circuits, suitable for use for example with an analog-to-digital converter (ADC).
FIGS. 4 and 5 illustrate analog integrator circuits suitable for use with an ADC. The integrator circuit illustrated in FIG. 4 includes a transconductance amplifier V, whose output is fed back via an integration capacitor Ci to its inverting input. A reference voltage V2 is applied to the non-inverting input of the amplifier V. A reference voltage V1 is applied to a series circuit consisting of an adjustable resistor R1 and a current source Q1 with a parallel parasitic capacitor Cp. The common connection point of the adjustable resistor R1 and the current source Q1 is connected to the inverting input of the transconductance amplifier V.
FIG. 5 illustrates an integrator in which the adjustable resistor is realized as a switched capacitor C1. This integrator therefore can be integrated in a space-saving manner.
The integrator circuits illustrated in FIGS. 4 and 5 are used, for example, in ADCs. The adjustable resistor R1 and the switched capacitor C1 are adjusted, depending on the voltage Vo at the output of the transconductance amplifier, in such a way that the current flowing through the adjustable resistor takes up the input current from the current source.
A problem with conventional analog integrator circuits occurs when the parasitic parallel input capacitance is large. For example, referring still to FIGS. 4 and 5, if the device providing the input signal to the ADC is a integrated photodiode PD, the photodiode PD generally has a relatively high parasitic parallel input capacitance Cp. As a consequence of the large parallel parasitic capacitance Cp and low input current, the ratio of the parallel parasitic capacitance Cp to the integration capacitance Ci (i.e., Cp/Ci) is about one-hundred. As a result, the amplification-bandwidth product is undesirably reduced by about two orders of magnitude.
When using a switched capacitance as the adjustable resistance, the bandwidth should be large enough, while at the same time the DC amplification likewise should be large, in order to ensure that the integrator circuit functions even at low frequencies. However, because these two requirements are contradictory, a compromise between them is necessary in order to achieve an acceptable bandwidth and an acceptable DC amplification.
Therefore, there is a need for an analog integrator circuit that provides the requisite bandwidth and DC amplification.
Briefly, according to the present invention, a voltage divider that includes a first and a second resistor and a current source with the parallel parasitic capacitance that together provide a second reference voltage. The connection point of the first and of the second resistor is connected to the inverting input of a transconductance amplifier.
The second resistor, which does not exist in the prior art, is dimensioned such that the ratios of the feedback network       j    ⁢          xe2x80x83        ⁢    ω    ⁢          xe2x80x83        ⁢    Ci        (          R2      +              j        ⁢                  xe2x80x83                ⁢        ω        ⁢                  xe2x80x83                ⁢        Cp              )  
are changed in such a way that a much higher amplification-bandwidth product is achieved.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.